Cathode for the winning of aluminum

ABSTRACT

In the winning of aluminum by the electrolysis of alumina dissolved in molten electrolyte, wherein the electrolysis current passes through the surface of a carbon cathode, the electrolysis current is equalized over the surface of the carbon cathode.

Wittner et a1.

[ Jan. 22, 11974 CATHODE FOR THE WHN a ALUMINUM Inventors: Hubert Wittner,

Ludwigshafen-Mundenheim; Kurt Lauei', Schriesheim, both of Germany Gehr. Giulini Gmhi-li, Ludwigshafen am Rhein, Germany Filed: net. 13, 1971 Appl. No.: 207,457

Assignee:

Foreign Application Priority Data Dec. 12, 1970 Germany....., P 20 61 263.9

US. Cl. 204/294, 204/243 R lint. Cl B01111 3/08, C22d 3/02 Field M Search 243 R 247 [56] References Cited UNITED STATES PATENTS 3,470,083 9/1969 Wrigge 204/243 R 3,582,483 6/1971 Sem 204/243 R 2,728,109 12/1955 Bonnot 204/294 X 3,514,520 5/1970 Bacchiega et a]. 204/243 R X Primary Examinerlloward S. Williams Assistant ExaminerD. R. Valentine Attorney, Agent, or Firm-George H. Spencer et a1.

[57] ABSTRACT In the winning of aluminum by the electrolysis of alumina dissolved in molten electrolyte, wherein the electrolysis current passes through the surface of a carbon cathode, the electrolysis current is equalized over the surface of the carbon cathode.

2 Claims, 4 Drawing Figures CATIIODE FOR THE WINNING OlF ALUMINUM BACKGROUND OF THE INVENTION The present invention relates to the winning of aluminum in an electrolysis cell. More particularly, the pres ent invention relates to a method of operating such acell and to a carbon cathode for such a cell, the cathode including carbon blocks having conductivities varying in steps from neighboring blocks.

In the winning of aluminum by the electrolysis of alumina (A1 dissolved in cryolite, electrolysis cells are used whose floor is constructed of an array of carbon blocks. These blocks collectively form the cathode. By means of a suitable bonding mass, for example unfired electrode mass, these carbon blocks are so connected to one another that an impermeable floor for collecting liquid metal is formed. The carbon blocks assembled into a cell floor have all had the same chemical and physical properties. Thus, it has not mattered which particular block was placed at any given position in a cell floor.

SUMMARY OF THE INVENTION It has been observed that when cells having the above-described carbon-block cathode floors are operated, certain disadvantages arise, especially in cells operated at high currents. For example, the central area of the cathodic floor grows upwards and this causes a lessening of its useful life. Also, the efficiency of utilization of electrial current is negatively influenced by curvatures in the bath surface due to magnetic field distortions.

It is an object of the present invention to provide cell operational method and cell structure for eliminating such disadvantages.

The achievement of this object by the present invention is based on the recognition that such disadvantages arise due to a gradient in the electrical resistance which is presented to current flowing through the abovedescribed cathodes built of carbon blocks of uniform electrical conductivity. This gradient exists in going from the middle of a cell floor cross to the borders of the floor. The path is of lower resistance from the molten aluminum, through the cathode, to a bus bar, when current enters the cathode floor at a border location, than when current enters the cathode floor at a central position. This has caused the electron flow to enter the molten aluminum from the cathode floor with a greater electron flow density at floor border regions than at floor central regions. This has meant that the carbon cathode has been loaded by the electrolysis current only scarcely at its middle but very strongly at its borders.

The above object as well as other objects which will become apparent in the discussion that follows are achieved, according to the present invention, by equalizing the electrolysis current over the surface of a carbon cathode present in the winning of aluminum by the electrolysis of alumina dissolved in molten electrolyte.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a plan view of the essential portions of a cathode block floor according to the invention.

FIG. 2a is a side view of one of the thirteen sets of cathode blocks of FIG. 1.

FIG. 2b is an end view of the set of FIG. 2a.

FIG. 3 represents the approximate equivalent electrical circuit of the structure of the invention.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the current density of the electrolysis current is the same over the entire cathode surface.

According to a preferred embodiment of the invention, the electrolysis current is equalized over the cathode surface by means of a cathode made of an array of carbon blocks of different electrical conductivities. These blocks are so arranged in the direction of cathode current withdrawal leads that a uniform current distribution is assured. In electrolysis cells with horizontally situated, continuous or divided cathode pins of metal for connecting the blocks with bus bars, the carbon blocks are so placed that the electrical resistance of the carbon blocks increases in steps from the cathode middle outwards in the direction of the pins. The steps are chosen to provide a substantially uniform current density over the cathode surface. The number of carbon blocks and their resistance parameters depends on the cell size and type and must be recalculated when considering a new aluminum electrolysis cell.

According to another embodiment of the invention, the cathode can be constructed of carbon blocks whose electrical resistances increase continuously outwards in the direction of the cathode leads. This eliminates the discontinuities caused by the stepwise increasing of resistance in the previous embodiment.

Referring now to the drawing, there is shown the essential structure of a carbon block floor I of an aluminum electrolysis cell. The view is down onto the plane of the floor. Bonding mass and the sides of the cell for containment of the molten aluminum and cryolite are not shown. The side view in FIG. 2a corresponds, for example, to the view of the carbon cathode floor appearing in FIG. 4 of the article entitled Aluminum in the McGRAW-HILL ENCYCLOPEDIA OF SCIENCE AND TECHNOLOGY, McGrawHill Book Company, New York (1966), at page 294.

The array of carbon blocks forming the floor includes blocks of three different resistivities. These three types of blocks are designated as blocks 3, blocks 4, and blocks 5. Along any of the thirteen continuous, steel, cathode pins 2, carbon blocks are arranged in electrical contact with the pins in the sequence block 3, block 4, block 5, block 4, block 3. The centers 3 of the blocks 5 are staggered alternately on one or the other side of the center line 9 of the floor; this somewhat smoothes out the effect of the resistance steps of one block to another along any one pin 2.

If an electrolysis cell provided with a cathodic floor of FIG. I is loaded with a current of 110,000 amperes, a uniform distribution of current over the surface of the floor is obtained when the resistivities at 900C, p p p of carbon blocks 5, 4, and 3 equal 9.0, 22.2, and 35.5 ohm'mm lmeter, respectively.

FIG. 3 shows a simplified equivalent circuit for the floor structure. Resistances R R and R represent the resistances of carbon blocks 3, 4, and 5, respectively. Resistance R represents the resistance of the steel pin 2 between points 10 and II in FIG. 2a, and resistance R represents the resistance of pin 2 between points 11 and 12. The voltage at points 13, I4, and 15, where the molten aluminum contacts blocks 3, 4, and 5, respec- Also, because the shapes of the blocks are the same, and the shapes of the steel sections, between points and 11, and 11 and 12, are the same, the following relationships also hold:

p5 pateel p4 psteel Pa where k and m are the shape factors for computing the resistances from the resistivities.

In an electrolysis cell provided with the cathodic floor of H6. 1 and loaded with a current of 110,000 amperes, the dimensions of the carbon blocks are 500 mm X 500 mm X 380 mm and the dimensions of the steel pins (cross section) are 140 mm X 140 mm. Carbon blocks with resistivities varying between 9.0 and 35.5 ohm mm meter are known. Manufacturers of such carbon blocks are, e.g., Siemens Griessheim AG and Conradty GmbH., Nurnberg.

Carbon blocks with defined resistivity can be produced by heating the blocks. The higher the temperature and the longer the heating time, the higher is the amount of graphite and the conductivity.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

We claim:

1. A carbon cathode for the winning of aluminum by the electrolysis of alumina dissolved in molten electrolyte, comprising an array of carbon blocks, with neighboring blocks in the array having different resistances, the difference in resistance between neighboring blocks being in a direction to equalize the current distribution between said blocks.

2. A carbon cathode as claimed in claim 1, further comprising a cathode current withdrawal lead in the array, with the block resistances increasing from the middle of the array outwards in the direction of the current withdrawal lead. 

2. A carbon cathode as claimed in claim 1, further comprising a cathode current withdrawal lead in the array, with the block resistances increasing from the middle of the array outwards in the direction of the current withdrawal lead. 